1. Field of the Invention
This invention relates generally to devices that emit electromagnetic radiation and, in particular, to vertical-cavity surface-emitting lasers (VCSELs).
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Laser beams are reflected to some extent from any surface contacted. If the reflected rays remain parallel (i.e., the angle of reflection equals the angle of incidence), the reflection is called “specular”. This means that the rays striking the surface are reflected from the surface according to the law of reflection, qi=qr. If the reflected rays are randomly scattered, the reflection is called “diffuse”. Specular reflections are produced by highly-polished, mirror-like surfaces whereas diffuse reflections result from rough, irregular surfaces (however, specular reflections can also be produced by rough surfaces when the size of the surface irregularities is less than the wavelength of the incident radiation). The distinction between a specular reflection and a diffuse reflection is not always clearly defined. Except for reflections from precisely constructed optical mirrors, all beams are to some extent divergent. In general, however, the rougher the reflecting surface, the greater will be the divergence (or diffuseness) of the reflected beam. FIG. 1 illustrates various types of laser reflections. FIG. 2 illustrates specular reflection from a polished mirror surface. FIG. 3 illustrates diffuse reflection from a corrugated surface.
Lasers have a wide range of industrial and scientific uses. There are several types of lasers, including gas lasers, solid-state lasers, liquid (dye) lasers, and free electron lasers. Semiconductor lasers are also in use. The possibility of amplification of electromagnetic waves in a semiconductor superlattice structure, i.e., the possibility of semiconductor diode lasers, was predicted in a seminal paper by R. F. Kazarinov, et al., “Possibility of the Amplification of Electromagnetic Waves in a Semiconductor with a Superlattice,” Soviet Physics Semiconductors, vol. 5, No. 4, pp. 707-709 (October 1971). Semiconductor laser technology has continued to develop since this discovery.
There are a variety of types of semiconductor lasers. Semiconductor lasers may be diode lasers (bipolar) or non-diode lasers such as quantum cascade (QC) lasers (unipolar). Semiconductor lasers of various types may be electrically pumped (by a DC or AC current), or pumped in other ways, such as by optically pumping (OP) or electron beam pumping. Semiconductor lasers are used for a variety of applications and can be built with different structures and semiconductor materials, such as gallium arsenide (GaAs).
Semiconductor lasers are typically powered by applying an electrical potential difference across the active region, which causes a current to flow therein. Electrons in the active region attain high energy states as a result of the potential applied. When the electrons spontaneously drop in energy state, photons are produced (to carry away the energy lost by the transition, so as to conserve energy). Some of those photons travel in a direction perpendicular to the reflectors of the laser. As a result of the ensuing reflections, the photons can travel through the active region multiple times.
Stimulated emission occurs when an electron is in a higher energy level and a photon with an energy equal to the difference between the electron's energy and a lower energy interacts with the electron. In this case, the photon stimulates the electron to fall into the lower energy state, thereby emitting a photon. The emitted photon will have the same energy as the original photon, and, if viewed as waves, there will be two waves emitted (from the electron's atom) in phase with the same frequency. Thus, when the photons produced by spontaneous electron transition photons interact with other high energy state electrons, stimulated emission can occur so that two photons with identical characteristics are present. If most electrons encountered by the photons are in the high energy state, the number of photons traveling between the reflectors tends to increase, giving rise to amplification of light and thus lasing.
The use of semiconductor diode lasers (both edge-emitting and surface-emitting) for forming a source of optical energy is attractive for a number of reasons. For example, diode lasers have a relatively small volume and consume a small amount of power as compared to conventional laser devices. Further, the diode laser is a monolithic device, and does not require a combination of a resonant cavity with external mirrors and other structures to generate a coherent output laser beam.
Additionally, semiconductor lasers may be edge-emitting lasers or surface-emitting lasers (SELs). Edge-emitting semiconductor lasers output their radiation parallel to the wafer surface, while in SELs, the radiation output is perpendicular to the wafer surface.
One type of SEL is the vertical-cavity surface-emitting laser (VCSEL). The VCSEL structure usually consists of an active (optical gain) region sandwiched between two distributed Bragg reflector (DBR) mirrors: a top, exit DBR, and a bottom DBR. DBRs are sometimes referred to as mirror stacks. The DBR mirrors of a typical VCSEL can be constructed from dielectric or semiconductor layers (or a combination of both, including metal mirror sections). DBRs or mirror stacks in VCSELs are typically formed of multiple pairs of layers often referred to as mirror pairs. The pairs of layers are formed of a material system generally consisting of two materials having different indices of refraction and being easily latticed matched to the other portions of the VCSEL. The number of mirror pairs per stack may range from 20-40 pairs to achieve a high percentage of reflectivity, depending on the difference between the refractive indices of the layers. A larger number of mirror pairs increases the percentage of reflected light (reflectivity).
When properly designed, these mirror pairs will cause a desired reflectivity at the laser wavelength. Typically in a VCSEL, the mirrors are designed so that the bottom DBR mirror (i.e. the one interposed between the substrate material and the active region) has nearly 100% reflectivity, while the top DBR mirror has a reflectivity that may be 98%-99.5% (depending on the details of the laser design). Of course, various laser structures may vary from these general properties.
High reflectivity (approaching 100%) at the bottom DBR mirror is generally desired in a VCSEL for two reasons. First, any portion of the optical field that “leaks” out the back of the bottom DBR mirror represents a power loss that reduces efficiency. This reduced efficiency may be so great so as to prevent the laser from operating at all (i.e. the efficiency goes to zero). A second reason why nearly unity reflection coefficient is desired for the bottom DBR mirror is related to the issue of optical feedback into the laser cavity.
From the standpoint of optics, any change in the index of refraction along the path of a light ray can be interpreted as a mirror, i.e., any change in the refractive index at an interface will cause some light to be reflected from the surface, rather than transmitted through. For the simple case of a ray of light normally incident to a semiconductor, the proportion of the light intensity reflected back from the interface is given by the equation:   R  =                              (                      n            -            1                    )                2            +              k        2                                      (                      n            +            1                    )                2            +              k        2            where n is defined as the index of refraction that describes the semiconductor optical properties, while k is the extinction coefficient. Any light that is not reflected by the bottom DBR mirror will be transmitted through the semiconductor for some distance (if k is zero for the substrate material, then it will be transmitted essentially without loss). Eventually, the light will impinge on the back surface of the semiconductor substrate, where it will undergo a specular reflection, according to the above equation. This specular reflection will cause some light to be reflected back towards the bottom DBR mirror, thereby forming a fabry-perot cavity (or etalon) between the back surface of the semiconductor substrate and the bottom DBR mirror. This etalon will inevitably couple with the laser cavity itself so as to affect the stable modes of the laser cavity. This effect may cause undesirable instabilities in the laser operation, such as mode-hopping, in which the laser optical field oscillates between two competing cavity modes. Other dynamic effects may also adversely affect the ability of the laser to be rapidly switched on and off, which may limit the application of the laser for some purposes, such as telecommunications or laser-based spectroscopic sensing.
For this reason, it is desirable to be able to limit the specular reflection of light from the back surface of the semiconductor substrate in a VCSEL.
One possible way to limit this specular reflection of light is to select an absorbing material for the semiconductor substrate so that any light that leaks out of the laser cavity through the bottom DBR mirror will be absorbed nearly completely before reaching the back surface of the substrate. However, the choice of substrate is already constrained by many factors in the laser design, such as lattice-matching to the DBR and active region materials, and electrical requirements such as the necessity to provide adequate doping to allow current to flow through the substrate material. In addition, since the substrate itself is often only several thousandths of an inch thick (typically 20 mils or 500 micrometers), it may be difficult to select a material with a sufficiently high absorption to reduce the amount of specular reflection to an acceptable level.
There is therefore a need for improved methods and apparatus for reducing specular reflection of light into the laser cavity of a VCSEL.